1. Field of the Invention
This invention generally relates to methods for estimating a vehicle's feed-forward lateral acceleration and, more particularly, to methods using a measured parameter of the vehicle's steering system to refine a feed-forward lateral acceleration estimation technique.
2. Description of Related Art
A vehicle develops an acceleration component lateral to the longitudinal axis of the vehicle when the vehicle begins to turn. During these situations it is normal in vehicles that provide an adjustability feature, to adjust the torque distribution between the wheels (front-to-back, side-to-side, etc.) in order to allow for effective handling of the vehicle. Lateral acceleration is one factor that is used in calculations that control the torque distribution amongst the wheels.
To enhance control response, it is advantageous to estimate the lateral acceleration of the vehicle from specific driver inputs. An estimated lateral acceleration may be used in a feed-forward control system to adjust for a condition caused by turning the vehicle, in combination with changing vehicle speed or alone, prior to the turn having a major affect on the stability of the vehicle. This estimated lateral acceleration is known as feed-forward lateral acceleration.
With reference to FIG. 3, a known feed-forward lateral acceleration estimation method is schematically illustrated. In this known method, the measured vehicle speed 10 is input into a gain table 12 to arrive at a signal 14 that is lateral acceleration per degree of steering angle. In multiplier 17 the measured steering angle 16 is multiplied with the signal 14 to provide a lateral acceleration signal 18, which is then converted to a magnitude value (by taking the absolute value 20) and its output is limited by passing this result through a saturation table 22 (to ensure the calculated value does not rise above actual vehicle cornering limits) to derive a normalized lateral acceleration signal 24. In multiplier 29, the normalized lateral acceleration signal 24 is multiplied by the sign of the steering angle, 26, to arrive at the estimated feed-forward lateral acceleration signal 28, which is used to control the front-to-rear and/or side-to-side torque applied to the vehicle wheels.
Unfortunately, in situations of low vehicle speed and tight turning, the known method for estimating feed-forward lateral acceleration become less accurate. More specifically, the feed-forward lateral acceleration is overestimated. It is believed that this overestimation is primarily due to the fact that the high turning angle (steering angle) tends to dominate the calculation. As a result, in a drive torque system utilizing feed-forward control, high rates of activation or shifting of drive torque may be implemented when they are, in fact, not required. Therefore, there exists a need in the art to correct the measured steering angle to compensate the feed-forward estimate of the lateral acceleration in low speed, tight turning situations.